1. Field of the Application
This application relates to the field of thermoelectric materials and more specifically to the field of thermoelectric nanocomposites.
2. Background of the Application
As the world's supply of fossil fuels continues to diminish, energy recovered from heat dissipated into the environment offers an abundant alternative fuel source. Temperature gradients are commonly produced by the environment (e.g., geothermal energy) or may be man-made by the countless systems that consume power (e.g., combustion engines, home appliances, etc.). These gradients are generally too small for conventional systems to adequately harvest energy from, but thermoelectric materials may have the ability to convert any temperature gradient into useful electricity. In order to harness this energy, an electrical current is created from the waste heat by the diffusion of charge carriers (i.e., electrons or holes) through the material from the hot side to the cold, or vice versa (i.e., the Seebeck effect). Traditional inorganic thermoelectric devices have garnered tremendous amounts of research due to their simple leg-type structure, high power density, and lack of noise pollution. However, only moderate improvements in conversion efficiency have resulted from this research. Typically, the resultant inorganic alloys contain heavy and expensive elements that require high processing temperatures and suffer from poor mechanical properties and toxicity issues. These issues have hindered the widespread use of the inorganic thermoelectric devices thus far.
Fully-organic, electrically conductive composites may provide an environmentally friendly, light-weight alternative to the traditional inorganic thermoelectric devices. Polymer-based materials are of interest because of their intrinsically low thermal conductivity associated with their composite matrix (≦0.2 W/(m·K)). Low thermal conductivity and high electrical conductivity are the ideal properties for efficient thermoelectric conversion as defined by the thermoelectric figure of merit (i.e. the dimensionless parameter “ZT”) which may be calculated by the following equation:ZT=(S2σT)/k(1)  (Eq. 1)where S, σ, k, and T are the Seebeck coefficient (or thermopower), electrical conductivity, thermal conductivity, and absolute temperature, respectively. The power factor (i.e. S2σ) is of primary concern when seeking to measure conversion efficiency in relation to lowering thermal conductivity. Typical semiconducting alloys have been shown to achieve power factors greater than about 2,500 μW/(m·K2), which is a ZT of about 1, at room temperature. For comparison, a thermoelectric material having a ZT of 4 may be equivalent to the efficiency of a home refrigerator.
Polymer nanocomposites, composed of carbon nanotubes (“CNT”) in a poly(vinyl acetate) latex may provide a suitable alternative to the traditional inorganic thermoelectric devices. As such, polymer nanocomposites have been developed that may have low densities, may not require complex manufacturing processes, and may have desirable mechanical properties (e.g., flexibility). Additionally, using a polymer emulsion (or latex) as the composite matrix may allow for conductive composites to be prepared from water under ambient conditions. An example emulsion may exist as an aqueous suspension of solid polymer particles with diameters between about 0.1 μm to about 1 μm. The larger polymer particles in the emulsion may exclude volume and force the smaller CNTs into the interstitial spaces that exist in-between the larger polymer particles. This excluded volume effect may be responsible for increasing the electrical conductivity of the network. This increase in electrical conductivity may be apparent even in polymer nanocomposites comprising small amounts of filler. Utilizing this segregated network approach, the electrical conductivity values of the polymer nanocomposites may be brought into the range of degenerate-semiconductor or metallic regimes.
For typical monolithic and inorganic semiconductors, increasing the electrical conductivity and/or the thermal conductivity may result in a decrease in the Seebeck coefficient. To decouple the Seebeck coefficient from the thermoelectric properties, water-based composites containing electrically connected, but thermally disconnected junctions may be used to create a small energy barrier for electron transport. This small energy barrier may deter low energy electron transport. In addition, this barrier may make it possible to increase electrical conductivity without decreasing the Seebeck coefficient. Additionally, the small energy barrier may be altered by the use of stabilizers needed to completely exfoliate the hydrophobic CNTs in water. The CNTs naturally form bundles due to Van der Waals forces between them. Varying the stabilizing agents may affect the electron transport across the CNT junctions. Surfactants, polymers, and inorganic nanoparticles have all been successfully used to exfoliate CNTs. It has been recently shown that intrinsically conductive polymer stabilizers, such as poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (“PEDOT:PSS”), may dramatically increase composite electrical conductivity. It has also been shown that CNT stabilizing, semiconducting molecules such as meso-tetra(4-carboxyphenyl) porphine (“TCPP”), may increase the Seebeck coefficient of these segregated network composites. Therefore, using a combination of multiple stabilizers to exfoliate carbon nanotubes may have the potential to simultaneously increase electrical conductivity, the Seebeck coefficient, and the power factor.
Consequently, there is a need for improved nanocomposite thermoelectric materials. Further needs include improved methods for making nanocomposite thermoelectric materials.